Sola Sound - Tonebender Mark II Professional  [schematic]

Discussion regarding early stompbox technology: 1960-1975 Please keep discussion focused and contribute what info you have...

Re: Sola Sound - Tonebender Mark II Professional

Postby Electric Warrior » 02 Dec 2017, 23:15

It's too little leakage for your bias setup, at least if you're going for the OC75/OC81D style thing. You could try 100k on Q1's base and 100k on Q2's collector. I hope that gets you close. They were biased to around 0.16V on Q2C. Hiss is not much of an issue in this voltage range. It's more prone to gating, but the gating can sound quite good.

That said, your Q1C looks like it could be in the right ballpark with the 10k, but I guess your battery voltage may be rather low.

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Re: Sola Sound - Tonebender Mark II Professional

Postby Rocket Roll » 02 Dec 2017, 23:24

Electric Warrior wrote:It's too little leakage for your bias setup...


Well, that's exactly the greatest part of the problem - those are the transistors that are supposed be selected to work well in a ToneBender by the good people in BYOC, right? If so, why aren't they?

Electric Warrior wrote:They were biased to around 0.16V on Q2C.


Great info, thanks!

Electric Warrior wrote:That said, your Q1C looks like it could be in the right ballpark with the 10k, but I guess your battery voltage may be rather low.


Forgot to mention - all measures taken with a laboratory supply at exactly 9.00V.
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Re: Sola Sound - Tonebender Mark II Professional

Postby Electric Warrior » 02 Dec 2017, 23:50

I guess it wasn't all that well known when they started out and they followed the general recommendations at that time. As you can see from the schematic, they had no chance to get this right.

Considering your supply voltage, Q1C is rather high. I'd definitely use a bigger resistor on its base.

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Re: Sola Sound - Tonebender Mark II Professional

Postby Rocket Roll » 03 Dec 2017, 08:45

Electric Warrior wrote:Considering your supply voltage, Q1C is rather high. I'd definitely use a bigger resistor on its base.


Thanks! Any recommendations? Which voltage do I aim for?
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Re: Sola Sound - Tonebender Mark II Professional

Postby Electric Warrior » 03 Dec 2017, 11:45

Around 90 to 95% of the supply voltage, so between 8.1 and 8.55 V would be dead on.
If the 100k on Q2C does the trick, A 100k for Q1B should be alright.

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Re: Sola Sound - Tonebender Mark II Professional

Postby Rocket Roll » 03 Dec 2017, 13:58

I'll try. One question, if I may...

Electric Warrior wrote:It's too little leakage for your bias setup...


I've got a small stash of germanium transistors that I can experiment with. Given the above numbers, what kind of leakage (IC and/or ICE0) am I looking for?
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Re: Sola Sound - Tonebender Mark II Professional

Postby Electric Warrior » 03 Dec 2017, 15:12

You could try around 0.35 mA. Whatever gets you in the right ballpark. Higher leakage should bring down the voltages on Q1C and Q2C, while the one on Q3C increases.

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Re: Sola Sound - Tonebender Mark II Professional

Postby Rocket Roll » 12 Dec 2017, 19:04

I've seen that SmallBear now stocks Amperex 2N281 transistors which are made in Holland, and according to this excellent read (https://sites.google.com/site/transisto ... rs/philips) they should correspond to Phillips OC72 type. I've ordered a set and received this:

Amperex 2N281 Flying Dutchman Smallbear.jpg

SmallBear The Flying Dutchman Transistor Set 12.2017.jpg

The Flying Dutchman Smallbear Set Schematic ToneBender MKii Pro.jpg


SmallBear measurements, as written on the bags:
Q1 - hFE 60, L(eakage) 125
Q2 - hFE 92, L(eakage) 158
Q3 - hFE 158, L(eakage) 181
Chinese transistor tester measurements:
Q1 - hFE 54, ICE0 67uA, ICEs 5uA
Q2 - hFE 96, ICE0 .20mA, ICEs 8uA
Q3 - hFE 186, ICE0 .33mA, ICEs 11uA

I've dropped them in a ToneBender MKII OC75 schematic, without any resistor substitutions, and got this:
In-circuit measurements, taken at 8.99V:
Q1, E 0.00, B 0.02, C 8.86V
Q2, E 0.00, B 0.08, C 0.15V
Q3, E 0.08, B 0.15, C 8.20V

The sound? It fuzzes allright, but let me put it this way: if I were someone who listened to all of the ToneBender lore and then got introduced to this sound, I would've thought that I'm simply not a fuzz person.

In this particular setup, they do not show more than 10% of the ToneBender potential. The sound is velcro-ish, octavey, gated. It is very present and "real", but not really pleasant.

If I haven't already completed two ToneBenders and somehow got crazily lucky with them (they sound very "Marshall in a box"-like, gainy and creamy, almost BigMuffish), I would've thought that ToneBenders are simply not my cup of tea, and that any zVex Fuzz Factory could do a number of such tricks, without the mythology attached.

So, do I change the transistor order and try to coax something out of this circuit as it is, or do I change the resistors - that is the question now...
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